The PFAS on the regulatory radar, such as PFOS, PFOA, and other related targets on typical laboratory lists, are anionic, or neutral. Therefore, the methods are optimized for anionic PFAS and feature standardized solid extraction using methanol containing ammonia, and weak anion exchange cleanup. However, many aqueous film forming foam (AFFF) active ingredients can be cationic or zwitterionic, and for these targets alternate approaches are needed. SGS AXYS had a cameo role in a new study on this important topic from Nickerson et al. 2020  out of Chris Higgins’ excellent Colorado School of Mines research group that I wanted to highlight in this post.
There has been some previous work from academic groups on research optimizing universal PFAS extractions. Munoz et al., 2018  studied multiple extraction options for anionic, neutral and zwitterionic PFAS that were relevant to AFFF-contaminated sites. In this study, Nickerson et al. used an optimized extraction, and multiple analytical techniques, including high-resolution Liquid Chromatography Quadrupole Time-of-Flight Mass Spectrometry (LC-QTOF-MS) and LC-MS/MS performed at the Colorado School of Mines and the total oxidizable precursor assay (TOP) performed by SGS AXYS.
TOP, developed by Houtz and Sedlak (2012) , is a semi-quantitative estimate of PFAS precursors. TOP works by oxidizing any PFAS that is amenable to attack by the hydroxyl radical into perfluorinated carboxylates that can be measured easily using standardized LC-MS/MS techniques. While the TOP assay provides a relatively straightforward and accessible approach to detecting the presence of non-target PFAS, it should never be regarded as a comprehensive mass balance. See book chapter on TOP authored by Bharat Chandramouli in Perfluoroalkyl Substances in the Environment: Theory, Practice, and Innovation for more . There are a number of structural limitations around conversion of all PFAS, measurement of ultra-short chain PFAS (<C3), and volatility. However, as long as these limitations are understood by data users, TOP is a valuable additional line of evidence. The SGS AXYS TOP methodology features reaction monitoring standards, isotope dilution methodology and careful standardization to minimize data variability.
Nickerson et al. used a sequential extraction featuring two extractions by basic (NH4OH) methanol followed by two rounds of extraction by acidic (HCl) methanol and compared this extraction with the standardized 3X basic methanol extraction (which is optimized for anionic and neutral PFAS). Three different soils, A, B and C, were tested. Given that most zwitterionic/cationic PFAS are not well characterized, and standards are not available, getting quantitative data is challenging. This study used a semi-quantitative external standard-based approach to calculate approximate, order of magnitude estimates using LC-QTOF. Extracts from the experiments were also analyzed by SGS AXYS using the TOP assay.
The paper (paywalled) has a good description of results with images that cannot be shared due to copyright. In summary, for the three soils tested via both TOP and the semi-quantitative analysis, one soil showed similar concentrations using TOF and TOP, and much higher than targeted analysis. In the other two soils, the TOF approach showed much higher concentrations than the TOP. It is impossible to estimate the source of the discrepancy because TOP reaction completeness for many cationic/zwitterionic PFAS are unknown, and the semi-quantitative approach uses response factors for a short PFAS list to estimate other PFAS based on relative area counts. Given that instrument responses of PFAS can vary by orders of magnitude depending on structure, this comparison is an indication of how challenging comprehensive PFAS analysis continues to be.
Importantly, the sequential extraction method provided significantly better extraction efficiencies for positively charged/zwitterionic PFAS than basic extraction alone, as indicated by the ESI+ runs. This work is an excellent addition to the body of work on improving the comprehensiveness of PFAS monitoring in AFFF sites.
- Nickerson A, Maizel AC, Kulkarni PR, Adamson DT, Kornuc JJ, Higgins CP. Enhanced Extraction of AFFF-Associated PFASs from Source Zone Soils. Environ Sci Technol. March 23, 2020.
- Munoz G, Ray P, Mejia-Avendaño S, Vo Duy S, Tien Do D, Liu J, et al. Optimization of Extraction Methods for Comprehensive Profiling of Perfluoroalkyl and Polyfluoroalkyl Substances in Firefighting Foam Impacted Soils. Analytica Chimica Acta, Volume 1034, November 30, 2018.
- Houtz EF, Sedlak DL. Oxidative Conversion as a Means of Detecting Precursors to Perfluoroalkyl Acids in Urban Runoff. Environ Sci Technol. 2012; 46(17):9342–9.
- Kempisty DM, Xing Y, Racz L. Perfluoroalkyl Substances in the Environment: Theory, Practice and Innovation. 1st ed. Boca Raton, FL: CRC Press; 2018. (Environment and Occupational Health.)